Surface light source devices which employ guide plates made of light scattering and guiding materials or transparent light guiding materials have been proposed and broadly applied to backlighting for LCD and other uses. Conventional surface light source devices are generally classified into a type featured by light supply from a side of light guiding means and another featured by light supply from behind light guiding means. The former is called surface light source device of side-light type.
FIG. 1 shows a partially broken perspective view of a LCD employing a conventional surface light source device of side-light type for backlighting. For the sake of illustration, exaggerated are formation pitch and depth of prism sheet and so forth.
Referring to FIG. 1, a guide plate 1 with emission directivity has a wedge-shaped cross section and is a light scattering guide plate or a transparent guide plate. The scattering guide plate 1 is a guide plate made of a known material which is capable of both light guiding and inside light scattering. The material is composed of matrix made of, for example, polymethylmethacrylate (xPMMA) and "substance of different refractive index" which is uniformly dispersed in the matrix. The term, "substance of different refractive index", is defined as substance having refractive index which is substantially different from that of the matrix.
A thicker side end face of the guide plate 1 provides an incidence end face 2 near to which a primary light source (fluorescent lamp) L backed by a reflector R is disposed. A reflector 3 is disposed along one major face (back face) 6 of the guide plate 1. The reflector 3 is made of a silver foil sheet with regular reflectivity or a white sheet with diffusible reflectivity. Illumination light is outputted from another major face (emission face) 5 of the guide plate 1.
A prism sheet 4 is a single-face prism sheet provided with a prismatic surface on one face, being disposed outside of the emission face 5. The prism sheet is orientated so that the prismatic surface is directed inward.
Referring to the broken illustration, an outer face 4c of the prism sheet 4 is shown as an even face. On the outside of the even face 4c, a LCD panel LP is disposed across a polarization separation sheet LS. The LCD panel has a well-known constitution comprising a liquid crystal cell, transparent electrodes and others, which are sandwiched between two polarizer plates having polarization axes crossing perpendicularly to each other.
The polarization separation sheet LS, an optical element in use growingly, is interposed between the prism sheet 4 and an inner polarizer plate of the LCD panel. The polarization separation sheet LS, an optical element in use growingly has a high transmissivity with polarization component having the same direction as the polarization axis of the inner polarizer plate while having a high reflectivity with polarization component perpendicular to the polarization axis of the inner polarizer plate.
The prismatic surface provided by the inner face of the prism sheet 4 has a great number of prismatic element rows. The prismatic element rows are orientated so as to be approximately parallel to the incidence end face 2 of the guide plate 1. As illustrated in a partially enlarged cross section, each prismatic element row is provided with a pair of slopes 4a, 4b forming a V-shaped valley. A first slope 4a is directed toward the incidence end face 2 of the guide plate 1 while a second slope 4b is directed toward a distal portion 7 of the guide plate 1.
Inclination angle .phi.a of the first slope 4a and inclination angle .phi.b of the second slope 4b are either equal (.phi.a=.phi.b or unequal (.phi.a&lt;.phi.b, usually) to each other case by case. The former corresponds to so-called symmetric prism sheet and the latter corresponds to so-called asymmetric prism sheet.
Light from the primary light L is introduced into the guide plate 1 and then is conducted towards the thinner end face 7 while being subject to scattering and reflection. Through this process, illumination light is outputted from the emission face 5 constantly.
Since light supply sideways with respect to the guide plate 1 causes the emission face 5 to emit light having a remarkably sharp directivity as known well, such a guide plate is generally called emission-directive guide plate.
The prism sheet 4 modifies a preferential propagation direction of illumination light, which has been outputted forward and obliquely from the emission face 5 owing to the above-mentioned directivity, toward a desirable direction (approximately frontal direction, usually).
Nevertheless conventional surface light source devices as described above have an advantage that thin structure is allowed, they are subject to serious problems against enlarging of emission area.
Cross section area of illumination flux is, at largest, approximately equal to area of the emission face 5 of the guide plate 1 to which the primary light source L supplies light while practicable size rising of the emission face 5 is limited. In particular, increased depth (distance from the incidence end face 2 and distal portion 7) not only requires the primary light source L to have increased power but also leads to difficulty that uniform brightness over the whole emission face is hardly realized.
In general, serious difficulty arises when depth is greater than about 10 inches. Under such situation, conventional LCDs with large display screen employ surface light source devices of a type such that a plurality of primary light sources are disposed behind light guide means instead of side light type.
Such arrangement makes it difficult to realize thin structure. And besides, difference in brightness is liable to appear between an area just above a primary light source and the other areas. To eliminate this, any element with strong diffusibility must be disposed. Such an element will lead to loss of light.
According to an prior art surface light source device, a pair of primary light sources are disposed at respective end faces of a large-size guide plate (so-called two-lamps type), which is nevertheless subject to disadvantages as follows.
(1) Depth will come to a large size, because space for accepting primary light sources must be prepared around both sides of the guide plate.
(2) It is difficult to employ structure in which an asymmetric prism sheet is incorporated Asymmetric prism sheets are designed under a condition that guide plates receive light supply from one side direction, as shown in FIG. 1, and light supply from both side directions will not provide good performance.
(3) It is also difficult to realize series arrangement including three or more primary light sources disposed along a direction of depth (i.e. direction of primary light supply).